Best advice I can give you, get all parties involved together for a chat.....
I wouldn't suggest that at all (sorry neko, no offence

), especially if your friend is the type to not be an open minded individual or he is deeply in love. Ask yourself this, is she worth it. Do you feel for her that much that you are willing to sacrifice your friendship with your best friend.
If answer is no, then you won't have a problem.
If answer is yes then you need to wait it out. Have a least that much respect for his feelings because that is what friends do. Also think about how you would feel if he was the one in love with the girl who was with you but didn't want to be. Just because she has told you, or you see that her feelings aren't there for your friend, doesn't mean it isn't going to be an issue if she dumps him and comes to you. My suggestion is if you two are right for each other, wait until there relationship isn't a sore spot for your friend. As he may feel deeply for her. Once he is over the break up blues, approach him and tell him about your feelings. Tell him from the start. Then go from there. If he gets upset then you know your risking your friendship and can make a decision between the two. If he isn't upset, then you are in the clear and all is well. Not to mention you gain trust and respect from him as he will see you waited to make this more comfortable for him.